The present invention relates to electric power generation and more particularly to a multi-cycle system for driving electric power generating turbines.
Water is used in power production systems because it is plentiful and inexpensive. The physical properties of water, whether in the ice, water or steam state, are well understood. For example, the amount of heat which must be taken in by a given mass of water to cause a change of state from liquid to gas (water to steam) is equal to the amount of heat which must be given up by the same mass of steam to cause the change of state from gas to liquid (steam to water). That heat is called the latent heat of vaporization. Conventional steam turbines efficiently convert the "mechanical" energy in steam to useful energy; however, the latent heat of vaporization in steam cannot be converted by conventional steam turbines to useful energy. That heat becomes waste heat and is transferred to the ultimate heat sink, the environment.
Conventional steam cycle commercial electric power generator systems have a low thermal efficiency, that is, the ratio of "heat converted to electric power by the system" to "total heat put into the system" is low. The thermal efficiency of a typical system is in the range of about 30 to about 35 percent. This low thermal efficiency is in large part due to the inability of conventional systems to convert the latent heat of vaporization of the steam into electric power.
Conventional steam cycle electric power generation plants burn fuels to make steam. The steam drives a turbine, which in turn drives an electric generator. The fuel may be fossil fuels such as oil and coal, or nuclear fuels, primarily uranium. For example, in a pressurized water reactor nuclear power plant, the uranium is "burned" in the reactor pressure vessel to heat water (reactor coolant), which is pumped in a high pressure "primary" closed loop through a heat exchanger, where it gives up heat, and then back to the reactor pressure vessel to be reheated and recycled. Water in a "secondary" loop (feedwater) is also pumped through the heat exchanger. In the heat exchanger, the feedwater in the secondary loop, which is at a relatively low pressure, is kept separate from the higher pressure reactor coolant in the primary loop. As the reactor coolant flows through the tubing in the heat exchanger, it gives up heat to the interior surface of the tubing. That heat is conducted through the tubing walls to the exterior surface of the tubing, where it is transferred to the feedwater.
As the molecules of the feedwater take in that specific quantity of heat from the exterior surface of the heat exchanger tubing, called the latent heat of vaporization, they flash into steam. The steam flows under pressure to the steam turbine, where, through pressure and temperature decreases, the steam gives up energy to the turbine blades (buckets) that translate the energy from the steam to rotational torque. That torque is conveyed to the turbine shaft, which drives the generator, producing electricity. As the steam exits the steam turbine, it flows at reduced pressure into another heat exchanger, called a condenser. There the steam gives up waste heat to the exterior surface of the tubing in the condenser, causing the steam to condense into water. The waste heat is conducted through the tubing walls to the interior surface of the condenser tubing, where it is transferred to circulating water. The circulating water takes away the waste heat to the ultimate heat sink, the environment (e.g., river, lake, atmosphere). Most of the waste heat is the latent heat of vaporization, which as was previously indicated, cannot be converted by conventional power generation systems to electricity. In fact, more than half of the heat produced by the combustion of fossil fuels or the fissioning of nuclear fuels, is a latent-heat-of-vaporization loss.
Some existing systems increase thermal efficiency by increasing the amount of heat transferred to the steam, rather than utilizing the latent heat of vaporization to decrease the amount of waste heat. This approach increases the thermal efficiency of the overall system because latent heat of vaporization is a smaller percentage of the total heat in the steam. However, this approach increases the expense of operation because it requires burning more fuel and adding more equipment.
Some steam cycle electric power generation systems have been proposed that use waste heat in a secondary gas cycle to generate power. However, these systems employ devices to increase the pressure (pumps or compressors) or to increase the temperature (superheaters) of the circulating gas. Some systems introduce a fuel to the gas turbine to generate more power. The cost of implementing such systems is high and none of these proposed systems have gained acceptance in the power generation industry.
Accordingly, there is a need for a more efficient utilization of heat in a steam cycle electric power generation plant to reduce the amount of fuel burned and to reduce the amount of waste heat dumped into the environment.